Method of fabricating uniformly distributed self-assembled solder dot formation for high efficiency solar cells

ABSTRACT

A method for fabricating a photovoltaic device includes performing a gettering process in a processing chamber which restricts formation of a layer of gettering materials on a substrate and forming a solder layer on the substrate. The solder layer is annealed to form uniformly distributed solder dots which grow on the substrate. The substrate is etched using the solder dots to protect portions of the substrate and form cones in the substrate such that the cones provide a three-dimensional radiation absorbing structure for the photovoltaic device.

RELATED APPLICATION INFORMATION

This application is related to commonly assigned U.S. patent applicationSer. No. 12/852,991 filed on Aug. 9, 2010, commonly assigned U.S. patentapplication Ser. No. 12/879,209 filed on Sep. 10, 2010, commonlyassigned U.S. patent application Ser. No. 13/161,163 filed on Jun. 15,2011 and commonly assigned U.S. patent application Ser. No. 13/220,816filed on Aug. 20, 2011, all incorporated herein by reference in theirentirety.

BACKGROUND

1. Technical Field

The present invention relates to photovoltaic devices, and moreparticularly to photovoltaic devices and methods for improvingperformance using cone-shaped pillars formed by etching with uniformlydistributed solder dots.

2. Description of the Related Art

Solar devices employ photovoltaic cells to generate current flow.Photons in sunlight hit a solar cell or panel and are absorbed bysemiconducting materials, such as silicon. Carriers gain energy allowingthem to flow through the material to produce electricity. Therefore, thesolar cell converts the solar energy into a usable amount ofelectricity.

By increasing the effective surface area for absorption, the solar cellcan be made more efficient. One way of achieving an increase in surfacearea includes use of a three-dimensional substrate. The threedimensional substrate may be formed by employing an etch mask createdusing lithography and etching pillars into the substrate. Forming anetch mask by lithography adds many steps and accrues additional cost toan already expensive process.

SUMMARY

A method for fabricating a photovoltaic device includes performing agettering process in a processing chamber which restricts formation of alayer of gettering materials on a substrate and forming a solder layeron the substrate. The solder layer is annealed to form uniformlydistributed solder dots which grow on the substrate. The substrate isetched using the solder dots to protect portions of the substrate andform cones in the substrate such that the cones provide athree-dimensional radiation absorbing structure for the photovoltaicdevice.

Another method for fabricating a photovoltaic device includes: placing asubstrate in an evaporation chamber; closing a shutter between agettering material source and the substrate; performing a getteringprocess in the processing chamber which restricts formation of a layerof the gettering material on the substrate; opening the shutter; forminga solder layer on the substrate; annealing the solder layer to formuniformly distributed solder dots which grow on a surface of thesubstrate; and etching the substrate using the solder dots to protectportions of the substrate and to form cones in the substrate such thatthe cones provide a three-dimensional radiation absorbing structure forthe photovoltaic device.

Yet another method for fabricating a photovoltaic device includes:placing a substrate in an evaporation chamber; closing a shutter betweena gettering material source and the substrate; performing a getteringprocess in the processing chamber which restricts formation of a layerof the gettering material on the substrate, the gettering processincluding: evaporating one or more of titanium, molybdenum and/orchromium in the processing chamber while maintaining a pressure below5×10⁻⁶ Torr for between about 180 to about 300 seconds with the shutterclosed; opening the shutter; forming a solder layer on the substrate;annealing the solder layer to form uniformly distributed solder dotswhich grow on a surface of the substrate; and etching the substrateusing the solder dots to protect portions of the substrate and to formcones in the substrate such that the cones provide a three-dimensionalstructure; and conformally forming at least one electrode and a diodestack on the cones to provide an active layer for providing current flowresponsive to incident radiation wherein the diode stack is disposedbetween two electrodes and at least one electrode is transparent, thediode stack including a p-doped layer, an intrinsic layer and an n-dopedlayer.

A substrate for a photovoltaic device includes a textured surface formedfrom silicon-based material. The textured surface includes a pluralityof cones uniformly distributed across the textured surface and isconfigured by etching from a top surface of the substrate, using aself-assembled solder dot mask evaporated on the substrate prior toetching. The cones are uniformly distributed as a result of gettering aprocess chamber prior to forming the solder dot mask. The cones have aheight/width ratio between about 1 to about 4, and the cones have adensity between 10⁸ to 10⁹ cones/cm².

Another photovoltaic device includes a substrate having a texturedsurface formed from silicon-based material, the textured surfaceincluding a plurality of cones uniformly distributed across the texturedsurface and is configured by etching from a top surface of the substrateusing a self-assembled solder dot mask evaporated on the substrate priorto etching. The cones are uniformly distributed as a result of getteringa process chamber prior to forming the solder dot mask. The cones have aheight/width ratio between about 1 to about 4, and the cones have adensity between 10⁸ to 10⁹ cones/cm². A continuous photovoltaic stack isconformally formed over the substrate and extends over and between theplurality of cones to form a three-dimensional structure. Thephotovoltaic stack is configured to transduce incident radiation intocurrent flow.

Another photovoltaic device includes a glass substrate having a texturedsurface including a plurality of cones uniformly distributed across thetextured surface and configured by etching from a top surface of thesubstrate using a self-assembled solder dot mask evaporated on thesubstrate prior to etching. The cones are uniformly distributed as aresult of gettering a process chamber prior to forming the solder dotmask. The cones have a height/width ratio between about 1 to about 4,with a height of between about 1 micron to about 4 microns and a widthbetween 500 nanometers and 2 microns. The cones have a density between10⁸ to 10⁹ cones/cm², the density is uniform across the substrate and isbetween about 1 cone per 1 micron and about 1 cone per 4 microns in alldirections. A continuous photovoltaic stack is conformally formed overthe textured surface of the substrate and extends over and between theplurality of cones to form a three-dimensional structure. Thephotovoltaic stack is configured to transduce incident radiation intocurrent flow.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a substrate in an evaporationchamber with a shutter closed during a gettering process in accordancewith the present principles;

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 having asolder metal layer formed thereon with the shutter opened in accordancewith the present principles;

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 having asolder metal layer formed on a dielectric layer in accordance with thepresent principles;

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 having asolder dot pattern formed by annealing the solder metal layer inaccordance with the present principles;

FIG. 5A is a scanning electron microscope (SEM) image showing solderdots formed using surface tension for organization;

FIG. 5B is a scanning electron microscope (SEM) image showing solderdots formed after a getting process showing a well-organized, uniformdensity and size in accordance with the present principles;

FIG. 6 is a cross-sectional view of the substrate of FIG. 4 showing thesolder dot pattern acting as an etch mask for etching pillars into thesubstrate in accordance with the present principles;

FIG. 7 is a cross-sectional view of the substrate of FIG. 6 showingpillars wet etched to form cones in the substrate in accordance with thepresent principles;

FIG. 8A is a scanning electron microscope (SEM) image showing conesformed using the surface tension formed mask of FIG. 5A;

FIG. 8B is a scanning electron microscope (SEM) image showing conesformed using the solder dot pattern formed after the gettering processfrom FIG. 5B, the cones being well-organized and of uniform density inaccordance with the present principles;

FIG. 9A is a cross-sectional view of the substrate of FIG. 7 showing anelectrode layer formed on the pillars/cones in accordance with thepresent principles;

FIG. 9B is a cross-sectional view of the substrate of FIG. 9A showing apin diode stack formed on the electrode layer in accordance with thepresent principles;

FIG. 9C is a cross-sectional view of the substrate of FIG. 9B showinganother electrode formed on the pin stack in accordance with the presentprinciples; and

FIG. 10 is a block/flow diagram of a method for fabricating aphotovoltaic device in accordance with illustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present principles provide photovoltaic devices and methods forfabrication that have improved absorption of incident radiation. Forhigh-efficiency silicon solar cells, a desired implementation includesthree-dimensional structures which maximize absorption of a lightspectrum and minimize reflection. The three-dimensional structures maybe formed by etching cone-shaped pillars which form the absorbingsurface. In one embodiment, cone formation may include the use of solderdots to pattern a wafer or substrate, for example, a silicon-basedsubstrate, for etching.

Solder or nanodot patterns may be self-assembled relying on surfacetension to form the dots during an anneal process. However, simplyletting the solder dots form on their own leads to areas of higher andlower density concentrations of dots as well as dots of varying size. Tosignificantly improve uniformity, the present principles provide methodsto assist in more evenly spacing the later-formed solder dots and toprovide more size uniformity. In this way, the spacings and sizes of thedots may be controlled to optimize manufacture and performance.

In accordance with preferred embodiments, a deposition process isemployed to remove impurities inside a processing chamber so that a morehomogeneous deposition of solder is possible. In one embodiment,titanium or other metal is evaporated in a chamber with a chambershutter closed. Then, solder is deposited on the substrate and annealedresulting in a uniformly distributed matrix of solder dots. The solderdots can be controllably grown to a desired size by controlling initialsolder deposition thickness.

When the solder dots are completed, three dimensional structures such aspillars or cones can be formed in the substrate. The three-dimensionalstructures may be employed for forming p-i-n diode layers or otherlayers for the formation of a photovoltaic cell or cells.

In other embodiments, multi-junction cells may be employed to achievesuperior carrier collection efficiency. Multi junction cells include twoor more cells stacked on top of each other. Any radiation transmittedthrough a top cell has a chance of being absorbed by a lower cell.

It is to be understood that the present invention will be described interms of a given illustrative architecture or substrate structure;however, other architectures, structures, substrate materials andprocess features and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Methods as described herein may be used in the fabrication ofphotovoltaic devices or chips. The resulting device can be distributedby the fabricator in raw wafer form (that is, as a single wafer orsubstrate that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case, the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case, the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips orphotovoltaic cells, ranging from toys and other low-end applications toadvanced computer products having a display, a keyboard or other inputdevice, and a central processor. The present embodiments may be part ofa photovoltaic device or circuit, and the circuits as described hereinmay be part of a design for an integrated circuit chip, a solar cell, alight sensitive device, etc.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a process for formingthree-dimensional structures (e.g., cone-shaped pillars) on a substrateis illustratively shown. The present process is maskless, low cost andmay be performed at low temperatures (e.g., below about 500 degrees C.).A substrate 12 may include silicon or a silicon-containing material andmay include glass, quartz, single (monocrystalline) silicon, etc. Whileother substrate materials may be employed, silicon based materialsincluding glass are preferred. A cleaning process may be performed onthe substrate 12. The cleaning process may include a solvent, such asacetone, isopropyl alcohol (IPA), deionized water or the like alone orin combination. Other solvents may be employed as well.

A dielectric layer 14 (FIG. 3) is formed on the substrate 12. Thedielectric layer 14 may include silicon oxide (SiO₂) on a general glasssubstrate or on a silicon wafer (e.g., single crystalline Si andmulti-crystalline wafers). For a glass substrate, the dielectric layer14 (SiO₂) may be formed to about 2 microns to 6 microns thick. Thedielectric layer 14 protects against rigid impurities such as Al₂O₃particles inside the glass which affect the etching processes of glassin making three dimensional structures. For Si substrates, thedielectric layer 14 (SiO₂) may be formed from about 50 nm to about 100nm using Plasma Enhanced Chemical Vapor Deposition (PECVD) at atemperature range of between about 250 degree Celsius to about 400degree Celsius. The dielectric layer 14 has been shown to enhance thedot formation on an oxide surface of the dielectric layer 14. However,the dielectric layer 14 may be omitted if a quartz substrate (12) isemployed which includes pure SiO₂ without any other impurities.

Substrate 12 (with or without the dielectric layer 14) is placed in aprocessing chamber 16, e.g., an evaporation chamber. A gettering processis performed in the chamber 16 with a shutter 18 closed between a source19 and the substrate 12. The gettering removes impurities which alsoresults in lowering the chamber pressure. The getter metals may includetitanium or other metals, e.g., molybdenum, chromium, etc.

Gettering is concerned with “clean-up” of residual gases by removing thegases from a vacuum vessel, such as the processing chamber 16. Getteringhere refers to chemical gettering. A chemical getter provides a pumpingaction by a chemical reaction where a chemically active gas combineswith a chemically active metal to form a low vapor pressure solidcompound. The chemically active metal can be either an element or analloy that, for convenience, can be called a getter metal or material,which may be employed to reduce or eliminate oxygen, nitrogen, carbondioxide, hydrogen, hydrocarbons, etc. from the chamber 16 by forming alow vapor pressure compound such that the active gas is permanentlyremoved from the vacuum chamber 16.

An evaporable getter may include a sublimation pump. A sublimation pumpoperates by heating the gettering material directly or indirectly byradiation from a filament or source 19 to cause sublimation. Oncesublimed into the vapor state, the gettering material is allowed tocondense on an internal array or on a portion of an inner surface of thechamber 16. As it condenses into a thin film, the gettering materialbecomes a pump for active gases and H₂.

In one embodiment, titanium is evaporated in the processing chamber 16.The processing chamber 16 includes a source crucible (or other sourcesuch as a filament) 19 that is isolated or blocked from the substrate 12during evaporation so that no getter materials are deposited on thesubstrate 12, but the chamber 16 and potentially the substrate 12 arecleaned by the gettering. In this illustrative embodiment, the titaniumis evaporated for 180 to 300 seconds at a chamber pressure of between1×10⁻⁶ to about 5×10⁻⁶ Torr. However, chamber pressure below 1×10⁻⁶ mayalso be employed.

The getter material evaporation is done with the evaporation shutter 18closed so that no getter materials are deposited on the substrate 12,but the process still removes impurities inside the chamber 16 sincegetter materials easily react with impurities that are or include, e.g.,carbon, oxygen and hydrogen.

Referring to FIG. 2, a solder metal layer 20 (FIG. 3) is deposited overthe substrate 12 or the dielectric layer 14. In the processing chamber16, the shutter 18 is opened up to permit deposition of the solder metallayer 20 (using a different source). The solder metal layer 20 mayinclude Sn, Pb, Sb, Bi, etc. or combinations thereof. In particularlyuseful embodiments, Sn solder or solders including Sn are employed. Thesolder metal layer 20 may include a thickness of between about 900Angstroms to about 2400 Angstroms, and more preferably around 1200Angstroms. The solder metal layer 20 may be formed by employing athermal evaporation method with a deposition rate of between about 1Angstrom per second to about 5 Angstroms per second. By controlling thethickness of the solder metal layer 20, later-formed solder dot size maybe controlled. FIG. 3 illustratively shows the substrate 12 having thedielectric layer 14 and the solder metal layer 20 formed thereon.

Referring to FIG. 4, an anneal process, such as a rapid thermal anneal(RTA), is applied to the structure such that solder metal layer 20 formssolder balls or solder dots 22. The solder balls 22 form uniformly(e.g., have a substantially uniform density) such that the dots coalesceand grow based on surface tension. The RTA may illustratively include atemperature of between 450 degrees C. to about 500 degrees C. for about10 seconds to about one minute. The temperatures and times may varydepending on the solder material and the desired results.

Referring to FIG. 5A, a scanning electron microscope (SEM) image showssolder dots 23 formed without a Ti getter process before Sn deposition.These dots were formed from a Sn layer having a thickness of about 1800Angstroms. The deposition rate was 1 Angstrom per second. The Sn layerwas then annealed at 450 degrees C. for 10 seconds. While the dots 23are dispersed across the surface of the substrate on which they areformed, there are regions of higher and lower concentration and thesizes of the dots have a wide size variation.

Referring to FIG. 5B, a SEM image shows solder dots 22 formed with theTi getter process before Sn deposition by evaporating titanium inaccordance with the present principles. The dots 22 were formed from aSn layer having a thickness of about 1800 Angstroms, as before. Thedeposition rate was 1 Angstrom per second. The Sn layer was thenannealed at 450 degrees C. for 10 seconds. The dots 22 are significantlymore uniform in density and size. Regions of different concentrationsand size variations were tightly controlled in accordance with thepresent principles.

Referring to FIG. 6, an etch process is employed to etch the substrate12 to form three-dimensional structures (e.g., pillars or cones) 24. Theetch process employs the solder balls 22 as an etch mask. For Si and/orglass dry etching, tetrafluoromethane (CF₄) plasma is preferablyemployed. Reactive ion etching parameters may include, e.g., a 30 minuteetch at 300 Watts of power and at a pressure of about 100 mTorr. Etchdepth may be approximately 1 to 4 microns, although other parameters anddimensions may be used and are effective.

The etch process is employed to transfer a pattern 25 of the solder dotsinto the substrate 12 (e.g., silicon or glass) utilizing a dry etch gaschemistry appropriate for etching substrate 12 with respect to thepattern of solder dots 22. The dry etch preferably includes a reactiveion etch (RIE) process so that the depth to width ratio is high forpillars or cones 24 formed as a result. In one embodiment, the substrate12 includes pillars 24 having a uniform density of between about 1 coneper micron and about 1 cone per four microns in all directions from apoint on the substrate 12.

Referring to FIG. 7, the solder metal layer 20 is preferably removedduring dry etching. If the solder metal layer 20 is not completelyremoved, a wet etch may be performed to completely remove the soldermetal layer 20, to remove the remaining portions of the dielectric layer14 and to further etch the pillars into cones 24. The wet etchingprocess further tapers the pillars/cones 24. As a result of etching withthe solder dot mask pattern 25, the substrate 12 includes a texturedsurface with cones 24 having a height between 1 to 4 microns and a widthof between 500 nm and 2 microns. A height/width ratio of the cones maybe between about 1 to about 4. A surface density of the cones 24 may bebetween about 10⁸ to 10⁹ cones/cm², and a side angle (A) from verticalmay be in the range of between about 45 and about 80 degrees. The wetetching process may include a dilute hydrofluoric (HF) etch, e.g.,HF:H₂O=1:50 for 10-20 minutes for a glass substrate, or a nitric acid(HNO₃) and HF etch for Si substrates.

Referring to FIGS. 8A and 8B, SEM images of etched glass substrates areshown for comparison. FIG. 8A shows pillars/cones 27 formed using theetch mask of solder dots 23 in FIG. 5A. FIG. 8B shows pillars/cones 24formed using the etch mask of solder dots 22 in FIG. 5B. The uniform anddense pattern provided in FIG. 8B yields a uniform dot array whichreduces the possibility of short circuiting the solar cell and increasesthe solar cell efficiency by providing higher collection efficiency.FIG. 8A includes a 2 micron scale 33, and FIG. 8B includes a scale 35 ofone micron.

The structure of FIG. 7 may be employed in a silicon-based solar cell.In one embodiment, the structure of FIG. 7 is employed to form a p-i-nstack structure thereon for a photovoltaic device. The cones 24 assistin increasing the surface area of absorption and provide for radiationtrapping therebetween. The cones 24 with larger than 1:2 (width toheight) aspect ratio are preferred for enhanced light absorption.

Referring to FIGS. 9A-9C and in particular to FIG. 9A, a process offorming an amorphous silicon solar cell is illustratively shown inaccordance with one embodiment. It should be understood that thesubstrate 12 employed in this process includes a transparent material,such as glass; however, silicon or other substrate materials may beemployed. In the present case, the substrate 12 has been processed in asame manner as described with respect to FIGS. 1-7.

A first electrode layer 32 is formed on cone 24 by a deposition process.The first electrode layer 32 may include a transparent conductivematerial such as a transparent conductive oxide (e.g., zinc oxide,indium tin oxide, indium zinc oxide, etc.), ultra-thin metal (e.g., 20nm or less in thickness) or other conductive structure. The depositionprocess may include a chemical vapor deposition process or othersuitable deposition process.

Referring to FIG. 9B, a p-i-n diode stack 34 is formed over the firstelectrode 32. The stack 34 preferably includes a first doped layer(p-doped layer), and intrinsic layer (i-layer) and a second doped layer(n-doped layer). The polarity of the doped layers may be reversed. Thestack 34 may be formed using a CVD or PECVD process. The stack 34provides active areas for absorbing radiation and converting theradiation into charge flow as is known in the art. A plurality ofdifferent materials may be selected for the layers in stack 34. In oneparticularly useful embodiment, the first and second doped layers mayinclude doped polycrystalline/microcrystalline silicon, and theintrinsic layer may include undoped amorphous silicon. The layers of thestack 34 may include monocrystalline silicon, multi-crystalline silicon(MC-Si), single crystalline silicon (SC-Si), amorphous silicon (a-Si),polycrystalline silicon (polysilicon), etc. Other materials are alsocontemplated.

Referring to FIG. 9C, a second electrode 36 is formed on the stack 34 toprovide a photovoltaic cell 100. The second electrode 36 may include atransparent conductive material such as a transparent conductive oxide(e.g., zinc oxide, indium tin oxide, indium zinc oxide, etc.),ultra-thin metal (e.g., 20 nm or less in thickness) or other conductivestructure. The deposition process may include a CVD, PECVD or othersuitable deposition process.

The cones 24 assist in increasing the surface area of absorption andprovide for radiation trapping between the cones 24. The structuresdepicted in FIGS. 9A-9C may be constructed to receive light with thecones 30 facing out (e.g., toward the light) or facing in. The cones 24increase the surface area and therefore the collection efficiency in anyincident light direction as compared with, e.g., a flat surface cell.

Referring to FIG. 10, a method for fabricating a photovoltaic device isillustratively depicted in accordance with particularly usefulembodiments. It should be noted that, in some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in FIG. 10. For example, two blocks shown in succession may,in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In block 202, a substrate is provided and cleaned by known cleaningmethods. The substrate may include materials such as, e.g., singlecrystalline silicon, multi-crystalline silicon, amorphous silicon,glass, quartz or other etchable materials. Depending on the material, adielectric layer may be formed on the substrate or as part of thesubstrate.

In block 204, the substrate is placed in a processing (evaporation)chamber. In block 206, a shutter in the evaporation chamber is closedbetween a source and the substrate. In block 208, a gettering process isperformed by evaporating a metal in the evaporation chamber. The metalmay include titanium although other metals may be employed (e.g.,molybdenum, chromium, etc.). A layer of the metal material should not beformed on the substrate (or dielectric layer). The metal is insteademployed to clean the chamber and potentially the substrate. In block212, evaporating the metal includes controlling duration of theevaporation. This includes evaporating the metal for between about 180to about 300 seconds with the chamber pressure up to 5×10⁻⁶ Torr orbelow.

In block 220, a solder layer is formed on the substrate. The solderlayer may include one of more of Sn, Pb, Sb, Bi or alloys thereof. Thisis performed in a same evaporation chamber as the gettering after theshutter is opened in block 221. A CVD or other process may be employedto deposit the solder layer. In block 222, the solder layer is formed tocontrol a size of the solder dots by controlling a thickness of thesolder layer. In block 224, the solder layer may be deposited at adeposition rate of between about 1 and about 5 Angstroms per second to athickness of between about 900 to 2400 Angstroms in thickness. Theprocess is well-controlled to accurately provide a solder metal layerthickness that translates to a size of the solder dots.

In block 230, the solder layer is annealed to form uniformly distributedsolder dots. The anneal process may include a rapid thermal anneal atbetween about 450 to about 500 degrees for between about 5 to about 60seconds.

In block 240, the substrate is etched using the solder dots to protectportions of the substrate to form cones in the substrate such that thecones provide a three-dimensional structure to enhance radiationabsorbing effects for the photovoltaic device. The etching process mayinclude a dry etch (e.g., RIE) which forms pillars followed by a wetetch to form the pillars into cones.

In block 250, a photovoltaic stack and electrodes are formed on thecones of the substrate. In block 252, an electrode and a diode stack areformed on the cones to provide an active layer for providing currentflow responsive to incident radiation. In block 254, a second electrodeis formed on the diode stack. The diode stack is disposed between thetwo electrodes. At least one electrode is transparent, and the diodestack may include a p-doped layer, an intrinsic layer and an n-dopedlayer. In block 260, processing continues to complete the photovoltaiccell. This may include forming additional layers or components asneeded.

Having described preferred embodiments of a device and method foruniformly distributed self-assembled solder dot formation for highefficiency solar cells (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method for fabricating a photovoltaic device,comprising: performing a gettering process in a processing chamber whichrestricts formation of a layer of gettering materials on a substrate,the gettering process including evaporating one or more of titanium,molybdenum and/or chromium into the processing chamber to removecontaminates of oxygen, carbon and nitrogen from the processing chamberby forming a layer of gettering material chemically bonded to thecontaminates on a sidewall of the processing chamber that is separatefrom the substrate; forming a solder layer on the substrate; annealingthe solder layer to form uniformly distributed solder dots grow on thesubstrate; and etching the substrate using the solder dots to protectportions of the substrate and to form cones in the substrate such thatthe cones provide a three-dimensional structure for the photovoltaicdevice, wherein the uniformly distributed solder dots provide auniformity in the cones ranging from 1 cone per micron to 1 cone per 4microns.
 2. The method as recited in claim 1, wherein the solder layerincludes one or more of Sn, Pb, Sb, Bi or alloys thereof.
 3. The methodas recited in claim 1, wherein performing a gettering process includesclosing a shutter between a gettering material source and the substrate.4. The method as recited in claim 3, wherein forming a solder layer onthe substrate includes opening the shutter before forming the solderlayer.
 5. The method as recited in claim 1, wherein performing agettering process includes evaporating a metal.
 6. The method as recitedin claim 5, wherein evaporating a metal includes maintaining a pressurebelow 5×10⁻⁶ Torr.
 7. The method as recited in claim 5, whereinevaporating a metal includes evaporating the metal for between about 180to about 300 seconds.
 8. The method as recited in claim 5, whereinevaporating a metal includes evaporating one of titanium, molybdenum andchromium in the processing chamber.
 9. The method as recited in claim 1,wherein forming a solder layer includes controlling a thickness of thesolder layer, wherein controlling the thickness of the solder layerimpacts size of the solder dots that are formed during annealing. 10.The method as recited in claim 1, wherein forming a solder layerincludes depositing the solder layer at a deposition rate of betweenabout 1 and about 5 Angstroms per second to a thickness of between about900 to 2400 Angstroms.
 11. The method as recited in claim 1, furthercomprising conformally forming at least one electrode and a diode stackon the cones to provide an active layer for a photovoltaic device. 12.The method as recited in claim 11, wherein the diode stack is disposedbetween two electrodes wherein at least one electrode is transparent,the diode stack including a p-doped layer, an intrinsic layer and ann-doped layer.
 13. A method for fabricating a photovoltaic device,comprising: placing a substrate in an evaporation chamber; closing ashutter between a gettering material source and the substrate;performing a gettering process in the processing chamber which restrictsformation of a layer of the gettering material on the substrate, thegettering process including evaporating one or more of titanium,molybdenum and/or chromium into the processing chamber to removecontaminates of oxygen, carbon and nitrogen from the processing chamberby forming a layer of gettering material chemically bonded to thecontaminates on a sidewall of the processing chamber that is separatefrom the substrate; opening the shutter; forming a solder layer on thesubstrate; annealing the solder layer to form uniformly distributedsolder dots which grow on a surface of the substrate; and etching thesubstrate using the solder dots to protect portions of the substrate andto form cones in the substrate such that the cones provide athree-dimensional radiation absorbing structure for the photovoltaicdevice, wherein the uniformly distributed solder dots provide auniformity in the cones ranging from 1 cone per micron to 1 cone per 4microns.
 14. The method as recited in claim 13, wherein the solder layerincludes Sn, Pb, Sb, Bi or alloys thereof.
 15. The method as recited inclaim 13, wherein performing a gettering process includes evaporating ametal.
 16. The method as recited in claim 15, wherein evaporating ametal includes maintaining a pressure below 5×10⁻⁶ Torr.
 17. The methodas recited in claim 15, wherein evaporating a metal includes evaporatingthe metal for between about 180 to about 300 seconds.
 18. The method asrecited in claim 15, wherein evaporating a metal includes evaporatingone of titanium, molybdenum and chromium in the processing chamber. 19.The method as recited in claim 13, wherein forming a solder layerincludes controlling a size of the solder dots by controlling athickness of the solder layer.
 20. The method as recited in claim 13,wherein forming a solder layer includes depositing the solder layer at adeposition rate of between about 1 and about 5 Angstroms per second to athickness of between about 900 to 2400 Angstroms.
 21. The method asrecited in claim 13, further comprising conformally forming at least oneelectrode and a diode stack on the cones to provide an active layer fora photovoltaic device.
 22. The method as recited in claim 21, whereinthe diode stack is disposed between two electrodes wherein at least oneelectrode is transparent, the diode stack including a p-doped layer, anintrinsic layer and an n-doped layer.
 23. A method for fabricating aphotovoltaic device, comprising: placing a substrate in an evaporationchamber; closing a shutter between a gettering material source and thesubstrate; performing a gettering process in the processing chamberwhich restricts formation of a layer of the gettering material on thesubstrate, the gettering process including: evaporating one or more oftitanium, molybdenum and/or chromium in the processing chamber whilemaintaining a pressure below 5×10⁻⁶ Torr for between about 180 to about300 seconds with the shutter closed, wherein the gettering processremoves contaminates of oxygen, carbon and nitrogen from the processingchamber by forming a layer of the gettering material chemically bondedto the contaminates on a sidewall of the processing chamber that isseparate from the substrate; opening the shutter; forming a solder layeron the substrate; annealing the solder layer to form uniformlydistributed solder dots which grow on a surface of the substrate; andetching the substrate using the solder dots to protect portions of thesubstrate and to form cones in the substrate such that the cones providea three-dimensional structure, wherein the uniformly distributed solderdots provide a uniformity in the cones ranging from 1 cone per micron to1 cone per 4 microns; and conformally forming at least one electrode anda diode stack on the cones to provide an active layer for providingcurrent flow responsive to incident radiation wherein the diode stack isdisposed between two electrodes and at least one electrode istransparent, the diode stack including a p-doped layer, an intrinsiclayer and an n-doped layer.
 24. The method as recited in claim 23,wherein forming a solder layer includes controlling thickness of thesolder layer, wherein controlling the thickness of the solder layerimpacts a size of the solder dots that are formed during annealing. 25.The method as recited in claim 23, wherein forming a solder layerincludes depositing the solder layer at a deposition rate of betweenabout 1 and about 5 Angstroms per second to a thickness of between about900 to 2400 Angstroms.